The diagram below shows the laccolith 75 million years ago (dike is not shown). Magma squeezed between rock layers, causing the layers above to dome upward slightly. The magma hardened beneath the surface. Move your cursor over the image below to see the laccolith today. The rock of the laccolith is more resistant to erosion than the sedimentary layers above and below, so it can be seen today as the cliffs of Crown Butte.

Move your cursor over the image.

Below: These two diagrams show how the Square Butte (laccolith), 50 miles east of Great Falls, formed. the diagram on the left (or top) show the laccolith, as well as the dike that supplied the magma. In the diagram on the right (or bottom), erosion has stripped off the overlying
sedimentary rocks, leaving the remains of the
laccolith and dike exposed at the
surface

The diagrams above
are from a special publication of the Montana
Bureau of Mines and Geology called
Profiles of Montana Geology
(publication #89). Although it depicts a butte
located 50 miles east of Great Falls, the
formation of Crown Butte was very
similar.

Laccoliths are a type of intrusive
igneous formation . . . . also called "plutonic
formations" or "igneous intrusions". This
means that they were formed as magma
cooled beneath the surface. With laccoliths
the magma moves into an area beneath the
ground, causing the overlying rock layers to
dome upward. After the magma becomes
rock, it is exposed as the less durable rock
above erodes away as shown in the
diagrams. The Little Rockies, located on the southern part of the Ft. Belknap Reservation are considered by many geologists to be a laccolith. However, the laccolith that caused the Little Rockies has a greater diameter and is deeper, and the exposed igneous rock resulted in a small mountain range, rather than a flat-topped butte.

Below: This photo, taken by Bob Rumney, shows Crown Butte as viewed from the southwest. Birdtail Butte (left) and Haystack Butte (right) are also intrusive igneous formation. However, they are not laccoliths.